Main skill for AD builds, because it's damage increases from your physical damage, so it's not that useful for AP build. Anyway, we are getting this on 1st level, for last hitting creeps, and harassing opponents, to make your laning phase easier.

Your bread & butter, as it does awesome amount of damages with AP build. Moreover, this skill hastes/slows your team mate/opponent attack speed by nice amount, which makes this skill more awesome. This will be your main skill during teamfights, and you have to max it out as soon as possible.

I can say, that this is one of the most entertaining skill for me in whole LoL. It gives you a nice escape mechanism, and moreover it's very useful while chasing your enemies + deals nice damage when going for AP build. It requires a good accuracy, when attacking enemies around good bunch of creeps, so it's not that easy to use, and needs some training.